The invention can be applied to a variety of applications.
One range of applications is solar powered systems where the fuel cell provides back up power to enable solar input variations to be tolerated. These solar input variations can be due to short term weather changes, but also can relate to sunrise and sunset time changes during the course of a year. A solar powered system may comprise a lighting system, such as street lighting or outdoor lighting.
The use of a regenerative fuel cell enables photovoltaic (“PV”) cell over dimensioning to be avoided. It also provides backup capacity to bridge several days of bad weather.
Another application is an oxygen generator for use in the administration of oxygen as a therapeutic modality.
For on-demand generation of oxygen, so-called oxygen concentrators have been developed in the past. The corresponding technology is well described in U.S. Pat. No. 6,551,384, for example. Currently, mobile oxygen generators are powered by batteries. On the highest oxygen generation setting, the user can experience a relatively fast drain of the batteries. When the batteries are depleted, the user needs to visit a location with a wall socket to enable the system to charge the batteries from the grid. Naturally the user can carry spare packs with him while on the move, but this adds weight.
For increased autonomy of a portable oxygen generator, a fuel cell can be used. A fuel cell system can provide relatively long periods of energy supply without connection to a wall socket to charge batteries. The lifetime of PEM fuel cells is an issue, particularly if the fuel cell is exposed to traces of CO as the membrane will suffer degradation. To extend the service life of a fuel cell stack, it can be over-dimensioned, or air filtering can be employed or a source of pure oxygen can be provided.
In both of the above mentioned examples, there is, based on the nature of the load, not a constant, flat production demand for electrical power from the fuel cell. In the lighting case, demand varies during the course of the night, as well as seasonally. In the oxygen generator case, the demand depends on the user. Other such variable demands will be apparent.
One way to design a system including a fuel cell is to design the system for the effect of having maximum service life and minimal on/off cycles. In the case of an oxygen generator, this requires a constant oxygen production for the fuel cell, and variations in patient's oxygen demand are to be produced on top of that.
Further, in an apparatus with a fuel cell and a battery, there are conflicting usage requirements to enable the battery life and the fuel cell life to be maximised. WO02/097909 discloses a hybrid power supply system in which a fuel cell is operated in a quasi-steady state mode. The fuel cell is operated at one of a discrete set of output currents in dependence on a state of charge of a battery. In particular, the fuel cell current setting is selected to meet current accepting limitations of a battery when charged using a constant voltage charge regime.
This document recognises that both the fuel cell operation and battery charging functions can each be optimised to extend lifetime. It is aimed at applications which have loads which are always fluctuating in demand.